Traditionally, a marine riser is provided with a tension system connected to the riser. The purpose of the tensioner system is to carry the weight of the riser, provide additional tensioning force to the riser and absorbing movement between the vessel and the riser within a preset and defined tensioner variation. The tensioning system will thus provide passive compensation for heave movements (wave action) and tide variations. The tensioner may comprise linear actuators in the form of hydraulic cylinder-piston arrangements being either directly coupled between the riser and the rig, or coupled via wire ropes and wire-sheave arrangement such as in the form of a jigger winch. Such systems are often referred to as respectively DRT, DAT—Direct acting riser/tension system, or WLT/WRT/MRT—Wire Line Tension/Wire Riser Tensioner/Marine Riser Tensioner.
Both methods have drawbacks.
Similar needs are present in operation of a drillstring, guideline, podline, or other downhole equipment such as wireline and coiled tubing operations, where the mentioned systems are typical means to compensate for the supported load and tensioning needs.
One possible solution for hydraulic riser tensioners employ hydraulic cylinders as their active element where the end portion of the hydraulic cylinder that includes the fluid cylinder part of the hydraulic cylinder is often connected to the rig main support structure and/or a movable support frame, the support frame being fixed to the vessel. The hydraulic cylinders piston rod end portion is often connected to the riser. The piston rod of the hydraulic cylinder is exposed to the environment when outside the cylinder. It is well known that the piston rod is attacked by chloride ions from the marine environment and high cyclic mechanical forces from the wave action. In addition grit and other particles coming through the air adheres to the often wet piston rod. The result is corrosion and increased wear of the piston rod, bearings, and seals. Such arrangements must frequently be maintained, and normally being located under the drill floor makes this operation expensive and time demanding.
Similarly for a drillstring compensator system using cylinders mounted in the top of the derrick is cumbersome to maintain in such aloft location and represent a weight that gives high center of gravity of the vessel affecting the variable load capacity.
A tensioner system being based on wireline assemblies arranged around the drill floor and substructure using a wire-sheave arrangement actuated by hydraulic cylinders arranged on the drill floor often results in a congested moon pool/drill floor area. Wire ropes and sheaves limits the access to areas where other drilling operation activities are conducted, and such systems represents a security risk for the personnel working on the drill floor. Additionally the total weight distribution on the drilling/production vessel is vertically higher because of the top deck/drill floor vertical mounted hydraulic cylinders and wire sheaves. Space requirements of such systems, also in the height, represent a challenge on the drill floor. Additionally such systems are limited in maximum capacity from a practical point of view by wire rope and sheave diameters. These systems are also maintenance intensive. For example handling the wire rope while doing cut and slip operations is difficult, especially for the larger wire rope sizes.
Existing offshore tension and compensator systems are normally based on hydraulics, set up to work with air pressure reservoirs, such as primary Air Pressure Vessels, APVs, and stand-by APVs. The primary APVs is arranged to generate the continuous pressure in accumulators in the tensioning system, and since these accumulators work with a constant operating cylinder area, the pressure will always be the same for a given primary APVs pressure. This pressure may be changed by a decrease valve venting the primary APV pressure or by filling additional gas/air/nitrogen into the primary APVs using an increase valve. The additional gas may be sourced from stand-by APVs or directly from a compressor unit.
For emergency situations, such as when a string cut off/riser disconnect is activated, the traditional system must be dimensioned to provide additional lift force (over dimensioned) for enabling lift off of the riser from the well head. An overcapacity of between 20-70% tension forces is normally provided for such emergency operation. The stand-by APV resource may be used if a rapid change in the mud weight in the riser is required. A rise in required mud weight increases the weight of the riser requiring increased pressure in the primary APVs. For example for drillstring compensators the pressure is increased when drilling deeper as additional drill pipe stands are connected to the drillstring.
An offshore drilling or production rig may need to operate in fast changing and challenging conditions, and large heave movements may dictate the variations in load capacity the tension systems must be able to compensate.
A first objective technical problem with existing systems is how to overcome or minimize, due to exposure to the sea water and drilling/production environment, the vulnerability to corrosion, wear and fatigue of the riser tensioner system.
A second objective technical problem is how to construct a riser tensioning system or compensator system that occupies minimal space in the tower or near the drill floor and in the moonpool area.
A third objective technical problem is how to make a compensator and or tensioning system more flexible and less dependent on the hydraulic reservoir of the accumulators and the reserve gas reservoir.
A fourth objective technical problem is how to lower the center of gravity of a drilling/production platform.
It is an object of the invention to provide solutions to at least one of the objective technical problems stated above.